Nanoporous linear polyolefin membranes and block copolymer precursors for same

ABSTRACT

A composition comprising a block copolymer that includes at least one polyester block and at least one linear polyolefin block, wherein the composition is in the form of a nano-structured, bicontinuous composite that includes a continuous matrix phase and a second continuous phase. The continuous matrix phase comprises the linear polyolefin block of the block copolymer, and the second continuous phase comprises the polyester block of the block copolymer. The composite may be treated to remove the polyester block, thereby forming a plurality of nano-pores.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/634,175 having a filing date of Oct. 23, 2012, which is a NationalStage application under 35 U.S.C. §371 of International Application No.PCT/US2011/028038 having an International Filing Date of Mar. 11, 2011,which claims the benefit of priority of U.S. Provisional Application No.61/312,922 having a filing date of Mar. 11, 2010.

STATEMENT OF FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DMR-0605880awarded by the National Science Foundation. The government has certainrights in the invention.

TECHNICAL FIELD

This invention relates to polymer membranes and processes for preparingsame.

BACKGROUND

Block copolymers are versatile hybrid materials that have been used inthe preparation of a wide variety of nano-structured materials. Theincompatibility of distinct chemical segments leads to nanometer-scaleself-organization, and thus utility as structure directing agents.

SUMMARY

In one general aspect, a process for preparing a polymer composite isdescribed that includes reacting a hydroxyl-terminated, linearpolyolefin polymer with a cyclic ester in the presence of a ring openingcatalyst to form a block copolymer having at least one polyester blockand at least one linear polyolefin block. The block copolymer is in theform of a nano-structured, bicontinuous composite. The compositeincludes a continuous matrix phase and a second continuous phase, wherethe continuous matrix phase comprises the linear polyolefin block of theblock copolymer, and the second continuous phase comprises the polyesterblock of the block copolymer.

As used herein, a “nano-structured, bicontinuous composite” refers to apolymer-polymer composite characterized by two continuous polymer phasesinterspersed throughout each other that exhibits compositionalheterogeneity on a nanometer (i.e., 1-500 nanometer) length scale.

In various implementations, the process may include treating thecomposite to selectively remove the polyester blocks of the blockcopolymer in the second continuous phase to form a plurality of pores.The composite may be treated by a chemical etchant. The pores may havean average pore diameter of about 1 to about 500 nanometers. The poresmay also have an average pore diameter of about 10 to about 50nanometers. In some embodiments, the resultant composite is in the formof a nano-porous membrane that may be a battery separator or waterpurification membrane.

Examples of suitable polyolefins include polyethylene and polypropylene.Examples of suitable cyclic esters include D,L-lactide, glycolide,caprolactone, menthide, and dihydrocarvide. When the cyclic ester isD,L-lactide, the resulting triblock copolymer includes polylactideblocks.

In another general aspect, a composition is described that includes ablock copolymer that includes at least one polyester block and at leastone linear polyolefin block in the form of a nano-structured,bicontinuous composite that includes a continuous matrix phase and asecond continuous phase. The continuous matrix phase includes the linearpolyolefin block of the block copolymer, and the second continuous phasecomprises the polyester block of the block copolymer. Examples ofsuitable polyolefins include polyethylene and polypropylene. Examples ofsuitable polyesters include polylactide. The composition exhibits goodmechanical properties, including modulus, tensile strength, andelongation at break.

In another general aspect, a composition is described that includes anano-structured, bicontinuous composite having a continuous matrix phasecomprising a linear polyolefin and a second continuous phase comprisinga plurality of nano-pores. The pores may have an average pore diameterof about 1 to about 500 nanometers. The pores may also have an averagepore diameter of about 10 to about 50 nanometers. In some embodiments,the composition is in the form of a nano-porous membrane that may be abattery separator or water purification membrane. The polyolefin can bepolyethylene or propylene.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a reaction scheme for synthesizing a polylactide-linearpolyethylene-polylactide (LEL) triblock copolymer.

FIG. 2 is a table reporting the molecular and thermal characteristicsfor the linear polyethylene (LPE) homopolymer, LEL triblock copolymers,and porous LPE samples prepared according to the Examples describedherein.

FIG. 3 is a 1H NMR spectrum of the polycyclooctene (PCOE) precursor usedto prepare the LEL triblock copolymers, with the two insets depicting aportion of the spectrum both with acetoxy end-groups (top) andhydrolysis (bottom) to afford hydroxyl end-groups.

FIG. 4 is a ¹H NMR spectrum of hydroxyl telechelic LPE fromhydrogenation of the PCOE, giving completely linear chains, with the endmethylene proton signal magnified for clarity in the inset. Measured intoluene-d₈ at 100° C.

FIG. 5 is a ¹H NMR spectrum of block polymer LEL [14-28-14] with theinset showing a magnified portion that accentuates the methylene protonsat the junction between the two components [H^(d);—CH₂—CH₂—O—C(O)—CH(CH₃)—] and the PLA end-group methine protons [H^(e);—O—C(O)—CH(CH₃)—OH]. Measured in toluene-d₈ at 100° C.

FIG. 6 depicts size exclusion chromatograms of the block polymersshowing the difference in elution volume between LEL [14-28-14] (

) and LEL [37-28-37] (

).

FIG. 7( a) presents DSC measurements for the unsaturated PCOE precursor(HO-PCOE-OH) (M_(n)=27.6 kg mol⁻¹, PDI=1.76), the saturated HO-LPE-OH(M_(n)=28 kg mol⁻¹, PDI=2.5) and triblock polymer samples LEL [14-28-14](M_(n)=55.7 kg mol⁻¹; f_(PLA)=0.38) and LEL [37-28-37] (M_(n)=102 kgmol⁻¹; f_(PLA)=0.62).

FIG. 7( b) presents thermograms from cooling the samples to accentuatethe relative crystallization exotherm magnitudes and show thecrystallization temperatures.

FIG. 7( c) presents DSC thermograms accentuating the T_(g) of the PLA inblock polymer samples. Heating and cooling rates were 10° C. min⁻¹, andthe samples were initially heated to 180° C. and isothermally annealedbefore analysis to homogenize thermal histories of the samples.

FIGS. 8( a)-(c) present SAXS analysis for the various samples showingthe broad scattering reflections associated with the bicontinuousdisordered structure. The primary peak appears in all cases to nestleagainst the beam stop at ˜0.05 nm⁻¹. FIG. 8( a): triblock copolymers inthe melt at 160° C. FIG. 8( b): triblock copolymers after cooling at˜20° C. min⁻¹ from the melt. FIG. 8( c): membranes at ambienttemperature after PLA removal.

FIGS. 9( a) and (b) present infrared spectra of the film prepared fromsample LEL [37-28-37] both FIG. 9( a) before and FIG. 9( b) afterremoving the PLA. The characteristic signal attributed to the carbonylfunctionality of the PLA (v=1750 cm⁻¹) is clearly absent after etching,suggesting complete PLA removal.

FIG. 10 is a scanning electron microscopy (SEM) microphotograph of afreeze-fractured LEL film after PLA etching (the length scale barrepresents 300 nm). Surfaces were sputter coated with platinum toprevent charging.

FIG. 11 presents SEM images at a variety of different magnifications forfreeze-fractured membrane from sample LEL [37-28-37] showing thedisordered bicontinuous nature of the structure where the narrowpore-size distribution and the homogeneity of the pore structure areaccentuated at high and low magnification, respectively. (≈2 nm Ptcoating).

FIG. 12 presents SEM images from the membrane derived from sample LEL[14-28-24] showing the similarly bicontinuous disordered morphologicalcharacteristics despite the significant difference in compositioncompared with the other sample described. (≈2 nm Pt coating).

FIGS. 13( a)-(b) represent nitrogen adsorption measurements on membranesmeasured at T=77K showing the adsorption (filled triangles) anddesorption (empty triangles) isotherms with the inset in each plotshowing the average pore size distribution calculated using the BJHmethod from the desorption data. FIG. 13( a): membrane from LEL[37-28-37]. FIG. 13( b): membrane from LEL [14-28-14].

FIG. 14 is a graph illustrating the pore size distribution for twodifferent LEL films (LEL [37-38-37] (filled triangles) and LEL[14-28-14] (empty triangles)) after PLA etching calculated from nitrogendesorption isotherms.

FIGS. 15( a)-(b) are graphs illustrating pore-size distributions fromnitrogen adsorption (filled triangles) and desorption (empty triangles)isotherms using the BJH method. FIG. 15( a) is derived from LEL[37-28-37]. FIG. 15( b) is derived from LEL [14-28-14].

FIGS. 16( a)-(b) are SEM images of porous LPE derived from LEL filmscast from 10 wt % tetralin solutions at 140° C. FIG. 16( a) is derivedfrom LEL [14-28-14]. FIG. 16( b) is derived from LEL [37-28-37]. (˜2 nmPt coating).

FIGS. 17( a)-(b) are stress-strain curves representing the results oftensile testing of block copolymer precursors (

) and membranes (- - - -) from samples LEL [14-28-14] (FIG. 17( a)) andLEL [37-28-37] (FIG. 17( b)).

FIGS. 18( a) and (b) are SEM microphotographs corresponding to afreeze-fractured LEL [14-28-14] film after PLA etching (the length scalebars represent 500 nm). FIG. 18( a) corresponds to the film prior toannealing, and FIG. 18( b) corresponds to the film after annealing at150° C. for 5 minutes.

FIGS. 19( a)-(d) are SEM microphotographs of surfaces exposed to (top,left and right) concentrated sulfuric acid (Fig[[s]]. 19(a) and Fig.(b)) and concentrated nitric acid for 24 h at RT (Fig[[s]]. 19(c) andFig. (d)) for porous sample derived from LEL [37-28-37]. Thebicontinuous morphology is well-preserved. (˜2 nm Pt coating)

FIG. 20 is a graph illustrating pore size distributions calculated usingthe BJH method from desorption isotherms for the membrane from LEL[14-28-14] before (curve (a)) and after (curve (b)) soaking inconcentrated hydrochloric acid at 50° C. for 24 h. The overall pore sizedistribution is minimally affected.

DETAILED DESCRIPTION

Polymer composites are prepared generally according to the reactionscheme shown in FIG. 1. The nano-structured nature of the compositeresults in films that exhibit good mechanical properties, includingmodulus, tensile strength, and ultimate elongation, that make themuseful in a variety of applications.

The polyester blocks (e.g., polylactide blocks) are incompatible withthe linear polyolefin block (e.g., polyethylene block). Theincompatibility results in microphase at some point after the blockcopolymer synthesis from the initial homogeneous state, and creating amulti-phase composite having a nano-structured, bicontinuousmicrostructure in which one of the phases includes the polyester blocks.

In some embodiments, the polyester blocks may be selectively removable,e.g., by chemically etching using base or acid. Removal creates aplurality of nano-sized pores. The pores are small (e.g., pore diameterson the order of about 1 to about 500 nanometers, or about 10 to about 50nanometers). In addition, the pores are characterized by a relativelynarrow size distribution, and are substantially homogeneouslydistributed throughout the film. These features make the nano-porousfilm particularly useful for applications such as separation membranes(e.g., battery separators). In general, the films are useful in avariety of applications, including separation membranes (e.g., batteryseparators), membranes for water purification, fuel cell membranes,catalytic reactors, nanotemplates, and the like. The nanoscopic,bicontinuous structure that results from the aforementioned processcontains interpenetrating domains that both percolate through the entirematerial. This co-continuity allows for one mechanically robust phase tosupport the entire structure and another percolating domain that endowsthe material with some specific functionality. Generating a nanoporousstructure by removal of the functional domain gives a material with apercolating pore structure. Since the pore size distribution is narrowand the pore structure permeates the entire film, such membranematerials are useful as battery separators.

Examples Materials

All bulk solvents were purchased from Mallinkrodt and used as receivedunless otherwise specified. Tetralin was purchased from TCI Chemicalsand was vacuum distilled prior to use. The second generation Grubbscatalyst was purchased from Aldrich and used as received. Bothcis-cyclooctene from Acros (95%) and cis-1,4-diacetoxy-2-butene from TCIChemical (95%) were distilled over CaH₂ prior to polymerizations.Tetrahydrofuran (THF) and toluene were passed through alumina columnsand thoroughly degassed. Purac provided the D,L-lactide (99%), which wasrecrystallized twice from toluene prior to being stored in a glove boxunder N₂ atmosphere. Sn(Oct)₂ from Aldrich was distilled using aKugelrohr apparatus and stored under N₂. The catalyst used inhydrogenation reactions was a silica-supported Pt catalyst supplied fromDow Chemical Company.

Characterization

¹H NMR spectra obtained using CDCl₃ as a solvent were measured on aVarian Inova 500 operating at 500 MHz, whereas those in toluene-d₈solvent were measured on a Varian Inova VI-300 operating at 300 MHz withvariable temperature capability up to 100° C. Size-exclusionchromatography (SEC) analysis was performed on two differentinstruments, depending on the relative solubility of the materials andtemperature capabilities of the instruments. Operating at a flow rate of1.0 mL min⁻¹ and 35° C. is a Hewlett-Packard (Agilent Technologies) 1100Series liquid chromatograph housing three PlGel 5 μm Mixed-C (PolymerLaboratories) columns with pore sizes of 500 Å, 1×10³, and 1×10⁴ Å withchloroform as eluent. The refractive index signal was recorded with aHewlett Packard 1047A refractive index detector. The other instrument,operating at a flow rate of 1.0 mL min⁻¹ and 135° C. with1,2,4-trichlorobenzene as eluent, is a Polymer Laboratories GPC-220liquid chromatograph holding three PlGel 10 μm Mixed-B columns andequipped with a refractometer used for samples with saturatedpolyethylene portions.

Small-angle X-ray scattering experiments were performed at the AdvancedPhoton Source (APS) at Argonne National Laboratories at Sector 5-ID-Dbeamline. The beamline is maintained by the Dow-Northwestern-DupontCollaborative Access Team (DND-CAT). The source produces X-rays with awavelength of 0.84 Å. The sample to detector distance was 5.65 m and thedetector radius is 81 mm. Scattering intensity was monitored by a Mar165 mm diameter CCD detector with a resolution of 2048×2048. Thetwo-dimensional scattering patterns were azimuthally integrated toafford one-dimensional profiles presented as spatial frequency (q)versus scattered intensity.

Differential scanning calorimetric (DSC) measurements were obtainedusing a DSC Q-1000 calorimeter from TA Instruments that was calibratedwith an indium standard. Samples were loaded into hermetically sealedaluminum pans prior to analysis. The thermal history of the samples wereall erased by heating the samples to 180° C. and isothermally annealingfor 5 min. The samples were then cooled at 10° C. min⁻¹ to −120° C.followed by a second heating cycle to 180° C. at a rate of 10° C. min⁻¹,all under a helium purge. Melting enthalpies were evaluated byintegration of the melting endotherm using TA Universal Analysissoftware.

Scanning electron microscopy (SEM) was performed on a Hitachi S-900FE-SEM operating at 3.0 kV accelerating voltage. Samples were preparedby fracturing small pieces of the films immediately after submerging inliquid N₂. Before imaging, the samples were coated with platinum using aVCR high-resolution indirect ion-beam sputtering system. The sampleswere coated for 10 min depositing approximately 2 nm of platinum.

Nitrogen adsorption/desorption was carried out at 77 K using anAutosorb-1 system. The specific surface area of the membranes wascalculated using the Brunauer-Emmet-Teller method (Brunauer, S.; Deming,L. S.; Deming, W. E.; Teller, E. J. J. Am. Chem. Soc. 1940, 62,1723-1732), while the pore-size distributions were determined using theBarret-Joyner-Halenda model (Barrett, E. P.; Joyner, L. G.; Halenda, P.P. J. Am. Chem. Soc. 1951, 73, 373-380).

General Procedure for Synthesis of HO-LPE-OH Macroinitiator

The procedure for preparing hydroxy-telechelic polyolefins byring-opening metathesis polymerization is generally described in (a)Bielawski, C. W.; Scherman, O. A.; Grubbs, R. H. Polymer 2002, 42,4939-4045, and (b) Pitet, L. M.; Hillmyer, M. A. Macromolecules 2009,42, 3674-3680. Briefly, 0.25 g (0.23 mL; 1.45 mmol) of the chaintransfer agent (CTA) cis-1,4-diacetoxy-2-butene was transferred to anair-free flask through a rubber septum along with 180 mL of THF. Thismixture was rapidly stirred and the temperature was maintained at 35° C.Using a syringe pump, 40 g (47 mL; 363 mmol) of cis-cyclooctene wereadded to the mixture over 1.5 h. Shortly (˜5 min) after starting thegradual monomer addition, 15 mg (18 μmol) of Grubbs 2^(nd) Generationcatalyst was added as a solution in 1 mL THF. After 6 h, the reactioncontents were slowly poured into 2 L of cold MeOH made slightly acidicwith 20 mL of 1M HCl (aq). The precipitated polymer was isolated anddried under reduced pressure at 40° C. for 2 days.

The entire yield was dissolved into 200 mL of THF and stirred at 0° C.for 6 h after adding 10 mL of a 0.7 M solution of NaOMe in MeOH (7 mmolNaOMe). The polymer solution was again precipitated into 2 L of slightlyacidic MeOH, isolated, and dried for 2 days, yielding 37.5 g (94%). ¹HNMR (CDCl₃, 25° C.): δ 5.40 (m, (E)-CH═CHCH₂CH₂—, backbone), 5.35 (m,(Z)—CH═CHCH₂CH₂—, backbone), 4.20 (t, (Z)—CH═CHCH₂OH), 4.10 (t,(E)-CH═CHCH₂OH), 2.05 (Z)—CH═CHCH₂CH₂— backbone), 1.95 (m,(E)-CH═CHCH₂CH₂ backbone), 1.30 (m, (Z)—CH═CHCH₂CH₂— backbone).

The hydroxy-telechelic PCOE (HO-PCOE-OH) (10.0 g; 45.4 mmol doublebonds) was dissolved in 150 mL cyclohexane and the solution was purgedwith bubbling argon for 20 minutes. A silica supported Pt/Re catalyst(1.0 g of 10%) was placed in the high-pressure reactor, which wassealed, evacuated of air, and refilled with Ar. The polymer solution wasadded to the reactor at which point hydrogen was introduced (500 psig)and the temperature increased to 90° C. The reaction mixture was stirredfor 24 hours, after which the solvent was removed and replaced with 150mL toluene. The catalyst was removed by filtering the solution at 110°C. and the solvent was again evaporated to afford 8.2 g of HO-LPE-OH(82% yield). ¹H NMR (toluene-d₈ 100° C.): δ 3.37 (t, —CH₂OH), 1.35 (s,—CH₂—, backbone).

General Procedure for Synthesis of LEL Triblock Polymers

The synthesis of one triblock is described, which is representative ofall samples where the D,L-lactide feedstock was adjusted accordingly totarget the desired polymer compositions. The concentration of LA waskept constant at 1 M. HO-LPE-OH (2.0 g; 0.14 mmol OH) was placed with astir-bar in a pressure vessel fitted with a Teflon screw-cap and Vitono-ring seal. This was transferred to a glove box, wherein D,L-lactide(2.5 g; 17 mmol), toluene (17 mL) and Sn(Oct)₂ (7 mg; 17 μmol) wereadded before sealing and removing from the box. The flask was immersedin an oil-bath at 110° C. for 6 h followed by precipitation into aten-fold excess by volume of MeOH. The isolated polymer was dried at 60°C. for 24 h to yield 4.2 g (93%). ¹H NMR (toluene-d₈ 100° C.): δ 5.10(bm, —C(O)CH(CH₃)O— backbone), 4.05-4.25 (m, —C(O)CH(CH₃)OH), 3.70-4.00(m, —H═CHCH₂OC(O)CH(CH₃)O—) 1.40-1.45 (—C(O)CH(CH₃)O— backbone),1.30-1.40 (—CH₂—, backbone).

General Procedure for Preparation of Block Copolymer Films andNanoporous Membranes.

The block copolymers were cast as films in aluminum pans by firstdissolving the polymer as a 10% solution in tetralin at 140° C. The hotsolution was transferred to the aluminum pan and the high temperaturewas maintained while the solvent slowly evaporated over 2 h. This wasinitially done to attempt to adopt an equilibrium microphase separatedstructure. The dry polymer film was kept at 140° C. for an additional 4h. The polymer films stuck to the aluminum. They were separated bydissolving the aluminum in a 4 M solution of HCl (aq). Melt-pressing ofthe block polymer precursors into cylindrical discs was done in a hotpress using molds with 13 mm diameter and 1 mm thickness.

The porous samples were prepared by submerging pieces of the blockpolymer (either bulk melt-pressed or solvent cast) in a 0.5 M NaOHsolution in 40% (aq) MeOH. The solutions were kept at 70° C. for 3 daysand the porous pieces were washed with slightly acidic MeOH (aq) andthen pure MeOH and further dried for 2 days at 60° C. in vacuo.Subsequent mechanical testing was performed on the solvent-cast films.

Results

The molecular and thermal characteristics for the LPE homopolymer, LELblock copolymers, and porous LPE samples, prepared as described above,are set forth in FIG. 2. A ¹H NMR spectrum of the polycycloocteneprecursor (PCOE) is shown in FIG. 3. A ¹H NMR spectrum of the hydroxyltelechelic LPE from hydrogenation of the PCOE is shown in FIG. 4. A 1HNMR spectrum of triblock copolymer LEL [14-28-14] is shown in FIG. 5.Size exclusion chromatograms of LEL [14-28-14] and LEL [37-28-37] areshown in FIG. 6.

A sample of LEL [37-28-37] was compression molded at 160° C. SAXSanalysis (FIGS. 8( a)-(c)) at 160° C. showed a broad signal with amaximum at 0.06 nm⁻¹ (d=105 nm) with no discernable higher-orderreflections consistent with a microphase separated structure lackinglong range order. The high degree of incompatibility between LPE andPLA, combined with low entanglement molecular weight for LPE, hinder theadoption of a well-organized mesophase. Annealing the samples up to 72 hat 160° C. did not appreciably increase the level of organization.Cooling from the melt to ambient temperature results in crystallizationof the LPE phase (FIGS. 7( a)-7(c)). SAXS analysis for either sample at25° C. (FIGS. 8( a)-(c)) gave virtually indistinguishable profilescompared to the 160° C. data, which is indicative of confined LPEcrystallization and consistent with behavior of other block polymers ofpolyethylene (i.e., hPB) and a highly incompatible component.

Exposure of molded LEL [37-28-37] samples to a 0.5 M solution of NaOHselectively removed the PLA, as confirmed gravimetrically and by IRspectroscopy (FIGS. 9( a)-(b)). An interconnected LPE scaffold with adisorganized pore structure was observed by scanning electron microscopy(SEM) (FIGS. 10-11). Etched LEL [14-28-14] samples show a similarlydisordered bicontinuous morphology (FIG. 12) after PLA removal despitecontaining significantly less PLA as compared to LEL [37-28-27].

Nitrogen adsorption analysis of nanoporous membranes derived from bothsamples showed type IV adsorption/desorption isotherms indicative ofmesoporosity (FIGS. 13( a) and (b)). Narrow pore-size distributions (BJHmethod; desorption isotherms) peaked at 24 nm and 38 nm for nanoporousmembranes from LEL [14-28-14] and LEL [37-28-37], respectively, withcalculated peak widths at half height equal to 3.5 nm and 11.1 nm (FIGS.14 and 15). Specific surface areas calculated for LEL [14-28-14] and LEL[37-28-37] derived membranes were 70 and 96 m² g⁻¹, respectively.

Thin (˜150 μm) films of the LEL samples were cast at 140° C. fromtetralin for tensile testing evaluation as described above. Thesesolvent cast films adopted the same disordered bicontinuous morphologiesas the molded samples, as determined by SEM (FIGS. 16( a) and (b)). Fromthe stress-strain curves of these samples (FIGS. 17( a) and (b)) thetensile toughness values were determined to be 1.54 and 4.91 MJ m⁻³ fornanoporous versions of LEL [37-28-37] and LEL [14-28-14], respectively.

Temperature-induced pore collapse is an important attribute in batteryseparators for preventing thermal runaway and minimizing the potentialfor ignition upon fortuitous anode/cathode contact. The DSC analysis ofthe nanoporous LPE membranes (FIGS. 2 and 7( a)-(c)) gave high meltingtemperatures (T_(m,PE)≈130° C.) and levels of crystallinity (˜60%) ascompared to typical values for hPB. Annealing the nanoporous LPEmembranes at 150° C. for 5 min causes pore collapse, as confirmed by SEManalysis (FIGS. 18( a) and (b)).

Chemical resistance to strong acids was evaluated by submerging sectionsof the LEL [37-28-37] derived nanoporous samples into concentratedsulfuric (@ RT), hydrochloric (@ 50° C.) and nitric (@ RT) acids for 24h. After rinsing and drying, >95% of the mass was retained in all cases.By SEM, there was little difference in the pore structure at the exposedsurface (FIGS. 19( a)-(d)) in both the sulfuric and nitric acid cases.After the HCl treatment the porosity and pore size distribution wereminimally affected according to nitrogen adsorption analysis (FIG. 20).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1-10. (canceled)
 11. A composition comprising a block copolymer thatincludes at least one polyester block and at least one linear polyolefinblock, wherein the composition is in the form of a nano-structured,bicontinuous composite that includes a continuous matrix phase and asecond continuous phase, wherein the continuous matrix phase comprisesthe linear polyolefin block of the block copolymer, and the secondcontinuous phase comprises the polyester block of the block copolymer.12. A composition according to claim 11, wherein the polyolefin ispolyethylene and the polyester is polylactide.
 13. A compositioncomprising a nano-structured, bicontinuous composite that includes acontinuous matrix phase comprising a linear polyolefin and a secondcontinuous phase comprising a plurality of nano-pores.
 14. A compositionaccording to claim 13, wherein the pores have an average pore diameterof about 1 to about 500 nanometers.
 15. A composition according to claim13, wherein the pores have an average pore diameter of about 10 to about50 nanometers.
 16. A composition according to claim 13, wherein thecomposition is in the form of a nano-porous membrane.
 17. A compositionaccording to claim 16, wherein the membrane is a battery separator. 18.A composition according to claim 16, wherein the membrane is a waterpurification membrane.
 19. A composition according to claim 13, whereinthe polyolefin is polyethylene.